1. Field of the Invention
This invention relates to a film forming method and a film forming apparatus. The present invention also relates to a semiconductor film and a photovoltaic device as well as to a solar cell, a sensor and an image pick-up device using such a photovoltaic device.
2. Related Background Art
Known methods for forming a crystalline silicon-based film include the cast method and other methods with which a film is grown from a liquid phase. However, such methods require the use of a process to be conducted at high temperature and hence is not particularly well adapted to mass production and cost reduction.
In an attempt for dissolving this problem and forming solar cells at low temperature, xe2x80x9cON THE WAY TOWARDS HIGH EFFICIENCY THIN FILM SILICON SOLAR CELLS BY THE xe2x80x98MICROMORPHxe2x80x99 CONCEPTxe2x80x9d, J. Meier et. al., Mat. Res. Soc. Symp. Proc., Vol. 420, p. 3, 1996 reports that a photoelectric conversion efficiency of 7.7% has been achieved for solar cells of a microcrystalline p-i-n structure formed on a substrate heated to 220xc2x0 C. by a glow discharge technique using a high frequency wave (110 MHz). There is also a report saying that a photoelectric conversion efficiency of 13.1% has been achieved for solar cells of a multilayer type using amorphous silicon and microcrystalline silicon.
While a microcrystalline silicon film obtained by a glow discharge technique as described in the above identified document operates excellently for photoelectric conversion, the reported film forming rate is far from satisfactory relative to the required film thickness and hence the disclosed method is not industrially feasible particularly in terms of the time necessary for the film forming process.
It is known that, in photovoltaic devices using a crystalline silicon-based film, carriers are generally prevented from moving freely to adversely affect the photoelectric performance of the photovoltaic devices by the influence of dangling bonds of silicon along the boundaries of crystalline silicon grains, the strain appearing on and near the boundaries of crystalline silicon grains, the imperfect crystallinity of silicon and other reasons.
Efforts have been paid to alleviate the above problems particularly in terms of improving the extent of crystallization. Such efforts include a low film forming rate, a heat treatment by irradiation of electron beams or laser beams or by using a lamp, and a film forming process of repeating a silicon film forming step and an annealing step in a hydrogen atmosphere. However, any of such techniques entails a long film forming process time and high cost.
In view of the above identified circumstances, it is therefore the object of the present invention to provide a silicon-based film and a photovoltaic device that show excellent photoelectric characteristics and can be formed at an industrially feasible rate.
According to the invention, there is provided a method of forming a film on a substrate by means of a plasma CVD process using a high frequency wave, the method comprising forming a resistance element made of a material different from that of the substrate and arranged on the electric path between the substrate and the earth for forming the film.
In another aspect of the invention, there is provided a silicon-based film formed on a substrate by means of a plasma CVD process using a high frequency wave, the silicon-based film being formed in the presence of a resistance element made of a material different from that of the substrate and arranged on the electric path between the substrate and the earth.
In still another aspect of the invention, there is provided a photovoltaic device comprising at least a plurality of silicon-based semiconductor layers of mutually different conduction types formed on a substrate, at least one of the silicon-based semiconductor layers being formed by means of a plasma CVD process using a high frequency wave in the presence of a resistance element made of a material different from that of the substrate and arranged on the electric path between the substrate and the earth.
In still another aspect of the invention, there is provided a film forming apparatus for forming a film on a substrate by means of a plasma CVD process using a high frequency wave, the film forming apparatus comprising a means for varying the insulation between the substrate and the earth.
Preferably, the high frequency wave shows a frequency preferably between 10 MHz and 10 GHz, more preferably between 13.56 MHz and 100 MHz. The source gas may not be decomposed sufficiently if the frequency is lower than 10 MHz. On the other hand, the electron temperature may not rise sufficiently and active species may not be produced satisfactorily if the frequency is higher than 10 GHz.
Preferably, the substrate is an electrically conductive substrate. When the substrate is an electrically conductive substrate, the substrate itself may be exposed and used as electrode.
Preferably, a means for producing a potential difference is provided between the substrate and the earth. Such a potential difference makes it possible to control the type of active species getting to the surface of the formed film.
Preferably, the resistance element is formed by arranging a material showing a volume resistivity not less than 1010 xcexa9cm at operating temperature on the electric path between the substrate and the earth. Any ion bombardment can be effectively controlled by selecting a volume resistivity not less than 1010 xcexa9cm. Preferably, the upper limit of the volume resistivity is set to 1021 xcexa9cm. Any electric current can hardly flow through the substrate to consequently charge the substrate with electricity and disturb the plasma distribution on and near the substrate if the volume resistivity exceeds 1021 xcexa9cm.
If the electric current flowing between the substrate and the earth during the generation of plasma is Ig when the substrate is grounded and the electric current flowing between the substrate and the earth during the actual film forming process is If, the film is preferably formed in a condition where the relationship of |If|xe2x89xa60.01|Ig|, more preferably the relationship of |If|xe2x89xa60.001|Ig|, is established. Any ion bombardment can be effectively controlled when such a relationship is established for |If|. For the above described reason, preferably |If| is not equal to 0, more preferably |If|xe2x89xa70.0001|Ig|.
While a resistance element is arranged between the substrate and the earth with a method or in a device according to the invention, it should be noted that Ig represents the electric current that flows between the substrate and the earth when plasma is generated under the condition that the resistance element is removed and the substrate and the earth are short-circuited so that no film will be formed under such condition (where an electric current of Ig flows). In other words, Ig represents an imaginary value. However, since it is possible to generate plasma while the substrate is grounded, the value of Ig can be determined with ease in any specific system.
Preferably, the power density of the high frequency wave is between 0.001 and 2 W/cm3 (high frequency wave power/plasma forming volume). Similarly, the pressure is preferably between 0.5 mTorr and 100 Torr.
Preferably, the value of |If| is changed during the film forming process. Particularly, it is preferable to increase the value of |If| during the film forming process.
Preferably, the silicon-based film shows a ratio of the diffraction strength of (220) due to X-ray diffraction or electron beam diffraction to the overall diffraction strength of not less than 30%.
Preferably, the photovoltaic device has at least a pin junction and at least the i-type semiconductor layer of the pin junction comprises a silicon-based film formed by a method according to the invention.